1. Field of the Invention
The present invention generally relates to a directional radio antenna, and more particularly, to a directional dipole antenna with adjustable reflector.
2. Description of Related Art
An antenna is a resonant device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. An antenna must be tuned to the same frequency band that the radio system to which it is connected; otherwise, reception and/or transmission will be impaired. The antenna size regularly refers as relative to wavelength. For example: a half-wave dipole, which is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle.
Gain and directivity are intimately related in antennas. The directivity of an antenna is defining a radiation of the RF energy directionally. It is quite intuitive that if the amount of radiating RF energy remains the same, but is distributed over less area, the apparent signal strength is higher in some particular direction. In another words, the directivity is the ability of an antenna to focus energy in a particular direction when transmitting or receiving energy. The attained increase in signal strength is the antenna gain. The gain is measured in decibels over either a dipole or a theoretical construct called an isotropic radiator. The qualitative relation between gain and directionality can be defined as gain=efficiency/directivity, where the antenna efficiency takes into account losses associated with the antenna structure and input terminals.
Another important characteristic of the antenna is a beamwidth, which characterizes the directivity of the antenna. The beamwidth is typically measured between the −3 dB points, i.e. the points on the main lobe where the signal strength drops off −3 dB (one-half) from the maximum signal point. The gain of the antenna is inversely proportional to the beam width: the narrower the beamwidth the higher the gain.
The antennas usually classified by the radiation characteristics on omnidirectional and directional antennas. Radio antennas produce a three-dimensional radiation pattern; however, for the description of the present invention it will be enough to analyze two-dimensional patterns from top or side projections. In the discussions below we will assume four different signals (A, B, C, D) arriving from different directions. In actual situations, of course, the signals will arrive from any direction, but we need to keep our discussion simplified.
The omnidirectional antenna radiates or receives electromagnetic waves equally well in all directions and does not favor any particular direction. One way to view the omnidirectional pattern is that it is a slice taken horizontally through the three dimensional sphere. FIG. 1 shows the pattern 102 for an omnidirectional antenna 101, with four cardinal signals. This type of pattern is commonly associated with verticals, ground planes and other antenna types in which the radiator element is vertical with respect to the Earth's surface. The key factor to note is that for receivers all four signals (or signals from any direction, from A, B, C, and D directions) are received equally well. For transmitters, the radiated signal has also the same strength in all directions.
Directional antennas focus energy in a particular direction. Directional antennas are primary used in applications where the coverage is preferable over some particular sector, and when one site needs to connect to only one other site or to multiple sites in same directional line, and omnidirectional coverage is not required. For example, point-to-point links are benefit from using directional antennas because it will minimize interference and maximize communications distance between these two sites. FIG. 2 shows the beam pattern 204 for antenna with a reflector and the beam pattern 203 for antenna without reflector.
Directional or reflector antenna, in one form or another, have been used since the discovery of electromagnetic wave propagation in 1888 by Hertz. Although directional antennas may take many geometrical configurations, in practical applications the most widely used shapes are the plane, corner, and curved reflectors, especially the paraboloid. The simplest type of reflector is a plane reflector. FIG. 3 and FIG. 4 show top and side views of a radiation pattern for a finite flat sheet reflector placed at quarter wavelength d 307 behind the dipole antenna 304. The azimuth radiation pattern of the vertical omni dipole antenna 304 without reflector composes a circle 301, and the elevation radiation pattern of this antenna with flat sheet reflector composes the shape 401. The approximation formula for the calculation of azimuth pattern 302 for dipole antenna with infinite flat sheet reflector is as follows:E(?)=sin(βd cos ?), where β=2p/?, and ? is a wavelength.
Azimuth radiation patterns of direct, reflected, and diffracted rays 303, direct and diffracted rays 308, and diffracted rays 306 also may be calculated analytically but require more extensive calculations; the corresponding formulas may be found in Balanis, Constantine, Antenna Theory: Analysis and Design, John Wiley & Sons, Inc. (1997).
To improve the collimation of the radiation pattern in the forward direction, the geometrical shape of the flat reflector must be changed in the way to prohibit radiation in the back and side directions. One configuration, which achieving this goal is a combination of two flat reflectors joined so as to form a corner. Because of the simplicity of such construction, the corner reflector found many applications. FIG. 5 and FIG. 6 show top and side views of a radiation pattern for a 90-degree corner reflector placed at quarter wavelength behind the dipole antenna 505, 604. The azimuth radiation pattern of the vertical omni dipole antenna 505 without reflector composes a circle 501, and the elevation radiation pattern of this antenna with a 90-degree reflector composes the shape 504, 502. Azimuth radiation patterns of direct and reflected rays 504, and diffracted rays 502 depend on such parameters as the spacing distance between the vertex of the reflector and the dipole antenna, the aperture of the corner reflector, etc.
The overall radiation parameters of the antenna with reflector may be further improved if the structural configuration of the surface of the reflector is optimized for better reflection. For example, it is well known in geometrical optics that if a beam of parallel rays is incident upon a reflector with parabolic shape, then radiation will converge in a focal point. In the same manner, if a source of the radiation is placed in the focal point, then the rays reflected by a parabolic reflector will come out in parallel rays. FIG. 7 and FIG. 8 show top and side views of a radiation pattern for a parabolic reflector 702, 802 placed at quarter wavelength behind the dipole antenna 703, 803. The azimuth radiation pattern of the vertical omni dipole antenna 703 without reflector composes a circle 701, and the elevation radiation pattern of this antenna with a parabolic reflector composes the shape 704. In the case of the circular parabolic reflector placed at quarter wavelength behind the dipole antenna, the approximation formula for the calculation of the width W of the main lobe half-power point is W=58°/(D/?), and the width WN of the main lobe between nulls is WN=140°/(D/?), where D is a diameter of the circular mouth of the reflector, and ? is a wavelength. The directive gain G of the antenna with parabolic reflector may be calculated with good approximation by the formula: G=4 p AK/?2, where A is an actual area of the mouth of the parabolic antenna; K is a correction factor of the order of 0.5–0.7 to compensate the non-uniform distribution of energy across the aperture due to tapering of the field; and ? is a wavelength.
A general concept of antenna reflectors and many particular applications have been discussed in a number of U.S. patents and publications.
The U.S. Pat. No. 4,663,632 “Extendable directional dipole antenna” discloses an extendable directionally adjusted dipole antenna suitable for use with motor vehicles. The antenna includes a vertical column having an extendable dipole arrangement at its upper end utilizing flexible actuators associated with a pair of reels where the actuators and associated telescoping antenna assemblies are simultaneously extended and retracted. An operating shaft for rotating the reels extends through the column and either manual or electric means rotate the shaft. The column is rotatable for directional adjustment, and under manual control the shaft extends through the vehicle roof permitting interior adjustments.
The U.S. Pat. No. 4,983,988 “Antenna with enhanced gain” discloses a combination of the plurality of vertically-polarized, omni-directional antennae having a reflector added to each one to limit the horizontal beamwidth to 90 degree and increase the gain to 16 dB. By utilizing four antennae and utilizing power combining hybrids to connect each of the two opposed antennae together, excess gain over a 10 dB omni-directional system will be obtained. The vertical beam width and physical height of the antenna are preserved with the increase in gain at the cost of an additional antenna complexity.
The U.S. Pat. No. 5,389,941 “Data link antenna system” discloses an antenna that employing the back radiation of a crossed-dipole to illuminate a parabolic cylindrical reflector. The crossed dipole is supported by a feed network mast, which simplifies the feed network and eliminates the need for other supporting structure and its electrical blockage. To provide an omni-directional radiation coverage, four of these antennas are located at the four quadrants, each covering one quadrant in the azimuth direction. The RF signal is fed through a single switch to the selected antenna to be radiated to the desired direction.
The U.S. Pat. No. 5,469,181 “Variable horizontal beam width antenna having hingeable side reflectors” discloses a broadband directional antenna having a central reflector plate, a dipole and at least one side reflector panel. The dipole is arranged on the central reflector plate for radiating a radio frequency signal, including a binary feed network having a microstrip transmission line and a collinear array of radiating elements. The side reflector panel is hinged to the central reflector plate for adjusting the horizontal radiation beamwidth of the radio frequency signal.
The U.S. Pat. No. 5,532,707 “Directional antenna, in particular dipole antenna” discloses a directional dipole antenna that is designed comparatively simply, and has improved electrical properties. It is provided that the symmetrical part of the antenna is made from the material of the reflector cut from the remaining material of the reflector wall, except for a connecting segment, which is cut preferably in the region of the immediate connecting point with the remaining material of the reflector wall and bent out relative to the plane of the reflector wall.
The U.S. Pat. No. 5,867,130 “Directional center-fed wave dipole antenna” discloses a directional center-fed half wave dipole antenna is constructed from a multilayer substrate having dipole antenna elements disposed on opposite surfaces of the multilayer substrate. An energy reflector is disposed on at least one side of the substrate and positioned adjacent to the dipole antenna elements that are fed by a center feed member that has a tapered width so as to provide the necessary impedance matching. A ground plane is disposed within the multilayer substrate, the elements of which are positioned on both sides of the center feed element.
The U.S. Pat. No. 6,198,460 “Antenna support structure” discloses an antenna support structure for at least three directional antenna sub-systems that can be planar antenna arrays. The antenna support structure comprises at least four panels adapted to support respectively one of the antenna sub-systems. The first two panels include a main panel and at least three secondary panels respectively adjacent to the main panel. The secondary panels can be respectively attached by hinge means to the main panel. In addition, the secondary panels can be individually adjusted in a predetermined angle to the main panel. The antenna support structure according to the invention can be particularly used in combination with wide-band printed dipole antennas for microwave and millimeter-wave applications.